1. Field of the Invention
The present invention relates to an optical head feed device for recording and reproducing information onto/from a recording medium such as a CD (compact disk), an optical disk, or a photomagnetic disk.
2. Description of the Prior Art
In optical head feed devices, signal tracks on the recording medium are spiral and hence the optical head is moved in a transverse direction of tracks on the recording medium at the time of recording and reproducing on the recording medium.
An example of a conventional optical head feed device will hereafter be described by referring to drawing.
FIG. 1A is a schematic top view showing a conventional optical head feed device.
As shown in FIG. 1A, an objective lens 1 is supported above an optical section 2 by a precision positioning actuator 3 made of an elastic material so that the objective lens 1 may be positioned precisely in the radial direction (i.e., tracking direction) of a disk D. The objective lens 1 and the optical section 2 are moved in the radial direction of the disk D by a coarse positioning actuator 7 including a carriage 4, a screw 5 and a stepping motor 6. The objective lens 1 is thus positioned coarsely. The precision positioning actuator 3 and the stepping motor 6 for the coarse positioning actuator 7 are controlled by a controller 8.
An optical head feed operation in the configuration heretofore described will hereafter be described.
Laser light emitted from the optical section 2 is passed through the objective lens 1 and focused on the recording face of the disk D. Reflected light from the recording face of the disk D is passed through the objective lens 1 and led into the optical section 2 again. From the optical section 2, a signal indicating how much the objective lens 1 deviates in the radial direction from a signal track engraved on the recording face of the disk D, i.e., a tracking error signal is outputted. On the basis of the tracking error signal, the controller 8 generates outputs for driving the precision positioning actuator 3 and the coarse positioning actuator 7. The precision positioning actuator 3 moves the objective lens 1 so that it may accurately follow one of a plurality of signal tracks recorded with a pitch of 1.6 .mu.m, for example, as in a compact disk. Since the range of movement of the precision positioning actuator 3 is narrow, the entire range of the disk D is covered by driving the coarse positioning actuator 7 to move the objective lens 1 and the precision positioning actuator 3 in the tracking direction.
A concrete control method for feeding the optical head will now be described by referring to FIG. 1B.
FIG. 1B is a block diagram showing a control system of a conventional optical head feed device.
In FIG. 1B, numeral 9 denotes a transfer function of a tracking system, 10 a low-pass filter, and 11 a lens position converting transfer function.
On the basis of the tracking error signal generated by the optical section 2 as described above, the tracking system transfer function 9 generates the drive output for the precision positioning actuator 3 after phase compensation. By using this drive output as input, the precision positioning actuator 3 moves the objective lens 1 so that it may follow the signal track even if the disk D is eccentric.
Since the signal tracks on the disk D are recorded spirally from an inner circumference to an outer circumference, the objective lens 1 following the signal tracks gradually deviates to an outer circumference. The deviation appears in the drive output for the precision positioning actuator 3 as a DC offset. Therefore, only a low-frequency component of the drive output for the precision positioning actuator 3 is taken out by the low-pass filter 10 and used to drive the coarse positioning actuator 7 to position the objective lens 1 nearly in the drive center of the precision positioning actuator 3. The lens position converting transfer function 11 is a transfer function for converting the displacement of the coarse positioning actuator 7 to a displacement of the objective lens 1. Eventually, the control system of this optical head feed device functions so that the error between the absolute displacement of the objective lens 1, which is determined by the displacements of the precision positioning actuator 3 and the coarse positioning actuator 7, and the signal track of the disk D may become 0.
By using the optical head feed device heretofore described, it becomes possible to make the objective lens 1 follow the signal tracks of the disk D over a wide range extending from an inner circumference to an outer circumference.
If there is a fluctuation in load and/or static frictional force of the coarse positioning actuator 7 for moving the whole including the precision positioning actuator 3, however, the coarse positioning actuator 7 in the above described conventional configuration does not begin to move until the low-frequency component of the drive output for the precision positioning actuator 3 becomes large. Therefore, the optical axis of the objective lens 1 might deviate from the signal track, resulting in degraded signal output. Further the actuator 7 might make a big movement resulting in moving the actuator 3 out of a controlled movable range limit of the actuator 3 and disabling the objective lens to track on or follow the signal track. Furthermore, since the control gain also fluctuates due to a fluctuation in load, stability becomes poor even if movement has been started. Furthermore, there is a risk of oscillation and runaway when the gain of the entire control system is increased to cope with a fluctuation in static frictional force.